If you are in hot countries all the time, you can get by with next to nothing, but if you are going to places that get at least a little cool at night, a lightweight fleece is an essential item. A fleece doesn't weigh much, you can cram it in anywhere, and it doesn't wrinkle. You can also subject it to all kinds of washing abuse--even Indian rock-banging laundry--and it will still last for ages. Good for chilly nights, for adding a layer, or for that early morning volcano hike.
If you buy one in the spring or summer, you can often get a fleece at a big discount. I picked up my pullover one at a Sears Land's End store two years ago for 10 bucks. They are on sale right now on that site for $15, men or women. There is usually a good sale at Sierra Trading Post and Backcountry.com's outlet store. Check the banner links on this page.